1. Field of the Invention
The present invention relates to a spindle motor servo system adapted for use in rotating a magneto-optical disc, where sample servo pits are formed, at a predetermined angular velocity in a magneto-optical recording/playback apparatus.
2. Description of the Prior Art
In recording desired data on a magneto-optical disc and reproducing the recorded data therefrom, the known techniques are generally classified into a method of sequentially recording the data on a continuous track, and another method of recording the data in accordance with servo pits formed previously at predetermined positions of a track on the disc.
FIG. 4A illustrates one surface of a magneto-optical disc to be recorded by the method of previously forming a plurality of sample servo pits P in each of sectors defined by circumferential division of the recording disc surface, and then detecting such sample servo pits P to produce various servo signals inclusive of a focus servo signal, a tracking servo signal and so forth for the magneto-optical disc. The sample servo pits P are arranged radially on the disc.
FIG. 4B is an enlarged view of an arrangement of sample pits P which comprise warbling pits P1, P2 for detecting a tracking error, and clock pits P3 for detecting a clock signal.
The clock pits P3 are detected as samples in a state where the magneto-optical disc is so driven that the rotation angle thereof is maintained constant, whereby the clock signal can be detected from reproduced RF data.
FIG. 5 shows a conventional spindle servo circuit for rotating the above-described disc D. In this example, the disc D is chucked by the rotary shaft of a spindle motor M and is driven to rotate together with an FG generator which detects the rotation rate.
Rotation pulses outputted from the FG generator F are supplied to a phase comparator 1 and then are compared with a signal obtained by demultiplying a reference signal of a crystal oscillator 2 by means of a frequency divider 3. The result of such comparison is fed to a low-pass filter 4 so that a phase difference error signal is formed. Meanwhile the output pulses of the FG generator F are supplied also to a frequency-to-voltage converter 5 to thereby detect the rotation frequency of the spindle motor via the comparator 6. Subsequently the rotation frequency signal is supplied to an adder 7 together with the phase difference error signal, and then the output of the adder 7 is supplied via a phase compensator 8 and a driver 9 to the spindle motor M.
According to such spindle servo circuit where the spindle motor M is rotated in synchronism with the signal of a predetermined period outputted from the crystal oscillator 2, an unshown fixed clock signal is reproduced from the magneto-optical disc D if the division ratio of the frequency divider 3 is set to a given value, so that data recording and reproducing operations can be performed in conformity with such clock signal.
However, in case the magneto-optical disc D is rendered eccentric due to any chucking deviation or when the track of the disc itself fails to be completely circular, it follows that some jitter is induced in the reproduced clock signal even if the spindle motor M is so controlled that its rotation rate is maintained constant. In relation to the eccentricity r1 based on the disc and the eccentricity r2 based on the chucking deviation, the amplitude of the jitter is expressed as EQU 2 (.DELTA.r1+.DELTA.r2)/j.omega.(where .omega.=2.pi.f)
And the frequency deviation with respect to the rotation frequency f of the disc is expressed as EQU f=(.DELTA.r1+.DELTA.r2)/j.pi.
In an ordinary disc chucking device, the frequency deviation f occasionally reaches 0.1 percent or so which is a non-trivial jitter component.
Therefore the clock reproducing circuit for the magneto-optical disc is equipped with a PLL (phase-locked loop) circuit synchronizable with the clock signal so as to absorb the jitter component. However, if there is employed such a PLL circuit that the capture range is narrow to stabilize the center frequency thereof, it becomes difficult to lock the PLL circuit in compliance with the jitter, hence raising a problem that a stable clock signal is not attainable.